Magnetic products of high aluminum iron alloys



MAGNETIC PRODUCTS F HIGIi ALUMINUM IRON ALLOYS Dusan Pavlovic and Karl Foster, Pittsburgh Pa, assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application August 1.6, 19 57 SerialNo.,678,524

s Claimsl (Cl.'148-121) This invention relates to a method of heat treating high aluminum-iron alloys to develop outstanding mag netic properties in .the resulting products.

Aluminum-iron alloys have heretofore been annealed in dry hydrogen at temperatures of 900 to 1050 C., and the investigators have reported that the magnetic properties of the alloy were independent of the temperature of treatment. In contrast to those reported results we have found that, surprisingly, the-magnetic properties of high aluminum-iron alloys annealed in hydrogen-are significantly affected when the annealing. temperature used is onthe order of about 12.00". C. Indeed, :the esulting products thereby obtained. have outstanding magnetic properties. That discovery forms the subject matter of our United States application Serial No. 678,- 539, filed August 16,-1.957.

While the process of our copending patent applica-. 11. 11 just referred to is entirely satisfactory as a method A temperature that may be as high as l200 to-1300 C. (2192 to 2365 F;) also is a process'variable that is not particularly, desirable from ,a commercial standpoint. Here too, if the temperature could besmaterially lowered, the attractiveness of the resulting process would be enhanced. I

, It is a major object of .-the present invention to pro vide a method for producing aluminum-iron alloys with outstanding magnetic properties that is economical; that e 2,937,115 c Patented May 17, 19n

Our invention generally is practiced by annealing a magnetic aluminum-iron alloy in air at an elevated temperature. Suitably the air anneal is carried out at a temperature 'of about 800 to 1000 C. and generally extends for at least 30 minutes and preferably for about 1 10 5 hours.- By way of example, we haveair-annealed many alloys of different compositions at 900 C. for 2 hours and obtained very satisfactory results.

After annealing, the product is cooled to a temperature of about 600 to 700 C.f Furnace cooling, or

coolingat a rate'ofabout 75'to 1150* C. per hour, is

satisfactory though a faster cooling rate such as would be experienced by air cooling also can he used if desired.

products.

Thesampleis cooled to a temperature approaching the ordering [transformation a temperature, i.e.. that temperature at which the structure of the alloy changes from FeAl to Fe A1, which' normally is about 550 C. for alloys that are used in this invention. At this point, the specimen is quenched to room temperature. As is apparent, to minimize induced stresses it is desirable to quench from as low a temperature as possible. Quenching can be carried out in any manner now known that will suppress ordering transformation whereby FeAl would transform to Fe Al. For example, the specimen may .be waterquenched .or .oil quenched.

"The materials to which our invention relates are mag.- netic aluminum iron alloys containing about 14 to 17.5 weight percent of aluminum. Aluminum-iron alloys are non-magnetic at an aluminum content that is greater than construed to include the magnetic compositions and is not an absolute value. Other alloying constituents and incidental impurities maybe present in varying amounts provided they do not deleteriously interfere with obtaining the described improved properties in the resulting We have discovered that these alloys in general can be air annealed successfully to produce products with good magnetic properties. That air annealing could be successful was unexpected, but it was particularly surprising to discover that at aluminum concentrations of '15 :to 17 percent, the properties of the resulting product were 'even-better thanthose obtained on the same alloy in a hydrogenfatmosphe're' at the same temperature. Indeed the fact that anoxygen atmosphere can -'be an improvement over'hydrogen still cannot satisfactorily be explained.

i "The alloys for use in the invention can be prepared by any procedure desired. A method that we have used does not depend on hydrogen atmosphere; and that requires no special equipment for its successful utilization.

Published reports indicate that aluminum-iron alloys with as much as 11.7 percent v,aluminurrrhave beenair annealed. That heat treatment was stated to result in serious degradation of the magnetic properties of the alloy. l:

In contrast to the reported results of other investigators, we have discovered that, surprisingly, certain aluminum-iron alloys can be heat treated in air to de-v 'We have also velop outstanding magnetic properties. I discovered that certain compositions within the broad range of alloys, that form the subject matter of this invention, upon treatment as herein described result in' magnetic properties that are on the order of the best propertiesv heretofore obtainable. Consequently, it is apparent that we have provided an economical heat treating process by whichhigh aluminum-iron alloys of noteworthy'magnetic properties can be produced and a process whereby particular products of outstanding mag- '.netic properties can be obtained at will.

involves vacuum melting a suitably pure iron, such as electrolytic flake iron, and then adding commercially 7 pure aluminum having a purity on the order of 99.99

' percent. 'The' aluminumis added to the iron melt under pared as follows: Electrolytic iron flake was melted in an I a blanket of helium to prevent loss of the aluminum.

Thereupon, the melt is cast either in avacuum of under 'a protective atmosphere to avoid loss of the aluminum and the uncontrolled introduction of impurities. The resulting ingot maybe hot or cold rolled to thedesired thickness. Generally, we roll the material to a thickness on the order of 1 to -30 mils because most commercial-applications require'that size; it should be'unde'rpracticed allow with an aluminum content of 14.7 Weight percent and others with greater amounts of aluminum were pre- 3 induction-heated, magnesium oxide crucible in a vacuum furnace at a pressure of 0.1 micron. Helium was then admitted to the furnace, and commercial aluminum bar was added to. the melt. The resulting melts were'cast andwhen theingots had'solidified, they were hot rolled at 1000 C. to 0.007 inch sheets. v Two sets of standard ring laminations were stamped from the 14.7 percent aluminum-iron (see table, below) while three sets of laminations were stamped from each sheet with an aluminum content of 15 or more percent. One set, or sample, of each composition was annealed at 900 C. for 2 hours in air; a second sample of each composition was annealed at 900 C. in dry hydrogen. The third sample of the higher compositions was annealed at 1200 C. in air for 2 hours. The annealed laminations were then furnace cooled to 600 C. Each sample was maintained at 600 C. for aboutlS minutes to be sure that equilibrium was achieved and was then quenched by being immersed in water. The products were then given routine D.-C. magnetic tests to determine the coercive force, residual induction and maximum permeability. The data obtained are shown in the following table:

Table A110 Ho Be Jllmnx Run Pegelent Heat Treat Atmosphere Oersted Gauss 14. 7 Air (900 C.) 0. 092 3, 400 18, 300 14. 7 Hydrogen (900 0.).-- 0.056 3,500 43, 600 15.0 Air (900 C.)..- 0.087 2, 500 16, 900 15.0 0.098 2, 350 15. 700 15. 0.087

V 16.8 Air (1,200 O.) 0.037

1 Taken from 100 oersted. 3 Not determined.

Considering the lamination sets that were air annealed and comparing them with those that were annealed in hydrogen, it is at once apparent that alloys having an aluminum content of 14 to 17.5 percent are not degraded in their magnetic characteristics by an air treatment. Indeed, even the poorer of these properties can be classed as good and when the property is considered as a function of its cost, the values obtained take on added significance.

The data on the alloys with a 15 percent and higher aluminum content are considered to be particularly surprising. Comparing the specimens that were annealed at the same temperature but in different atmospheres (i.e 1 vs. 2; 3 vs. 4; 6 vs. 7; 9 vs. 10; and 12 vs. 13), it can be noted that in each instance the product obtained in the air anneal was characterized by as good as or better properties (coercive force) than the hydrogen annealed product. There is no conclusive accounting for these outstanding results even after the fact.

These data also show that an anneal at 1200 C. in air can result in satisfactory properties. At 15 percent aluminum, air annealing at 1200" C. resulted in a better coercive force than in the hydrogen annealed product. On the other hand, the data of the 1200 C. air anneal point up still another anomaly of this discovery. Whereas if the hydrogen anneal were used, a treatment at 1200 C. would result in marked improvement as compared with the results at 900 C., .the use of air as the annealing atmosphere brings about just the opposite result because, as shown by the data in the table, the products obtained at 900 C. are superior to those obtained at 1200 C.

From the foregoing data and discussion it is apparent that our discoveries make it possible to produce magnetic aluminum-iron materials of noteworthy magnetic prop erties by a procedure that is farmore economical than those known heretofore. These discoveries are particularly surprising not only because of the economical processes which they provide'in face of published indications that it could not be accomplished, but also because they demonstrate another fact about aluminum-irons and their unpredictable behavior, thereby contributing to the knowledge about these alloys. These products can be substituted for the commercially available siliconiron and nickel-iron alloys that are used in magnetic applications.

In accordance with the provisions of the patentstatutes,

' we have explained the principle of our invention and have described what we now consider to represent its best embodiment. However, we desire to have it understood that the invention may be practiced otherwise than as specifically described.

We claim: t

1. A method for providing improved magnetic properties of coercive force, remanence, and maximum permeability in an aluminum-iron alloy sheet which comprises heating in the presence of air for at least one half hour a magnetic alloy sheet consisting essentially of 14 to 17.5 weight percent of aluminum and the remainder iron at a temperature of from 800 C. to 1200 C., and then quenching the resultant annealed sheet from a temperature that is above that at which an ordering transformation occurs, wherein the structure normally changes to Fe Al, to-about room temperatures, the quenching suppressing the ordering transformation.

2. A method for providing improved magnetic properties of coercive force, remanence, and'maximum permeability in an aluminum-iron alloy sheet which comprises heating a magnetic alloy sheet consisting essentially of 15 to 17 weight percent of aluminum and the remainder iron, in air ata temperature of about 800 to 1000 C. for about 1 to 5 hours, cooling the resultant annealed sheet to about 600 and then quenching the sheet to about room temperature, the quenching suppressing the ordering transformation.

3. A method according to claim 2 in which said alloy is in the form of a tape of about 5 to 30 mils in thickness.

References Cited in the file of this patent UNITED STATES PATENTS Nachman Aug. 6, 1957 OTHER REFERENCES 

1. A METHOD FRO PROVIDING IMPROVED MAGNETIC PROPERTIES OF COERCIVE FORCE, REMANENCE, AND MAXIMUM PERMEABILITY IN AN ALUMINUM-IRON ALLOY SHEET WHICH COMPRISES HEATING IN THE PRESENCE OF AIR FOR AT LEAST ONE HALF HOUR A MAGNETIC ALLOY SHEET CONSISTING ESSENTIALLY OF 14 TO 17.5 WEIGHT PERCENT OF ALUMINUM AND THE REMAINDER IRON AT A TEMPERATURE OF FROM 800* C. TO 1200* C., AND THEN QUENCHING THE RESULTANT ANNEALED SHEET FROM A TEMPERATURE THAT IS ABOVE THAT AT WHICH AN ORDERING TRANSFORMATION OCCURS, WHEREIN THE STRUCTURE NORMALLY CHANGES TO FE3AL, TO ABOUT ROOM TEMPERATURES, THE QUENCHING SUPPRESSING THE ORDERING TRANSFORMATION. 